One of the principal steps in the process of manufacturing a printed circuit board is the electroforming or electroplating of a conductor such as nickel in holes drilled through the board. For example, the basic process of producing a unitube printed circuit board involves the drilling of holes in an epoxy fiber glass board/aluminum laminate, electroforming nickel in the holes, and then dissolving the aluminum to leave the nickel tubes protruding from the board. These protruding tubes can then serve as the basis for the attachment of leads to the board by means such as welding. Thus, the thickness, hardness and ductility of the tubes must be favorably disposed to produce acceptable welds.
While various methods and devices have been developed to produce unitubes having uniform plating thickness across the board together with tubes having the necessary weldable qualities, each present solution appears to have certain inherent limitations and/or drawbacks. For example, it has been found that cathode shields, such as described in U.S. Pat. No. 3,429,786 have the disadvantage of using considerable space in the electroforming tank and require assembly to the cathode before the plating operation. Also, these shields tend to restrict agitation of the electroforming solution across the face of the cathode and thus may increase pitting of the electrodeposit. Such shields provide shielding for the redundant side only. Therefore, after the initial plating time, the operator must place the two plating racks face to face in order to prevent overgrowth of the circuits and to continue building up the circuits. This causes inferior plating on the surface due to the low current density. Attempts to improve the plating uniformity and unitube ductility by varying the constituents in the nickel sulfamate plating solution and by varying the conditions of temperature, time, pH, and current density did not provide satisfactory results.
Examples of unitube manufacturing processes which might utilize the present invention are disclosed in U.S. Pat. Nos. 3,370,351, 3,396,459, 3,426,427, 3,429,036, 3,429,037, 3,429,038, 3,431,641, 3,462,832 and 3,508,330 plus U.S. Ser. No. 329,798 filed Feb. 5, 1973, now U.S. Pat. No. 3,819,430, and U.S. Ser. No. 374,747 filed June 28, 1973, now U.S. Pat. No. 3,855,692.